Modified lithium niobate composition and devices utilizing same

ABSTRACT

The phase-matching temperature for different frequencies of electromagnetic waves in LiNbO3 is increased by the addition of MgO to the melt. Variation of this temperature in successive portions of the grown crystal is minimized. The resulting product is usefully incorporated in nonlinear optical devices such as second harmonic generators and parametric oscillators designed for operation at elevated temperatures at which radiation damage is annealed out.

United States Patent Bridenbaugh et al. 5] May 23, I972 [541 MODIFIEDLITHIUM NIOBA'IE 1 References Cited COMPOSITION AND DEVICES UNITEDSTATES PATENTS UTILIZING SAME 3,528,765 9/1970 Fay et a1. ..330/4.5 [72]Inventors: Paul Michael Bridenbaugh, Piscataway;

John Robert Carruthers, Murray Hill; Franklin Richard Nash, Griggstown,all of Primary xaminerRoy Lake Assistant Examiner-Darwin R. HostetterAn0meyR. J Guenther and Edwin B. Cave [57] ABSTRACT Assignee: BellTelephone Laboratories, Incorporated,

Murray Hill, Berkeley Heights, NJ.

Filed: Feb. 16, 1971 Appl. No.: 1 15,467

U.S. Cl. ..307/88.3, 321/69 Int. Cl.

The phase-matching temperature for difierent frequencies ofelectromagnetic waves in LiNbO is increased by the addition of MgO t0the melt. Variation of this temperature in successive portions of thegrown crystal is minimized. The resulting product is usefullyincorporated in nonlinear optical devices such as second harmonicgenerators and parametric oscilla- Field Of Search ..307/88.3; 321/69;331/107 tors designed for operation at elevated temperatures at whichradiation damage is annealed out.

6Clains,4DrawingFigures 2 oayo t [O /A WAS) x \--l/3 0.25 e 5 020CONGRUENT MELT 2 STOICHIOMETRIC y 0.|5 MELT 3 CQNGRUENT MELT PLUS M900.05

Illlllllllllll -s0 -40 o 40 80 I20 I60 200 240 280 PHASE MATCHINGTEMPERATURE (c) MODIFIED LITHIUM NIOBATE COMPOSITION AND DEVICESUTILIZING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention Theinvention is concerned with nonlinear devices designed for altering thefrequency of traversing electromagnetic waves as, for example, byharmonic generation or parametric oscillation.

2. Description of the Prior Art Nonlinear" devices are popularlyutilized in the frequency conversion of electromagnetic radiation in andnear the visible spectrum. Such devices are based on the nonlineardependence of polarizability on wave amplitude. Types of devices includeharmonic generators, e.g., second harmonic generators (SHG), and avariety of parametric devices which usually result in a downshifting offrequency. Operation of oscillators used on the latter principle mayinvolve two frequencies as in the degenerate case or three frequenciesas in the nondegenerate case. Interest in this technology is tracedlargely to the advent of the laser and to the recognition that thenumber of fundamental frequencies now available and likely to beavailable from such sources is limited.

An early recognized difficulty in parametric devices arises fromfrequency dispersion, i.e., the velocity dependence on frequency forelectromagnetic wave energy generally observed in any real medium. Whilethe nonlinear relationship does result in frequency conversion, thedifi'ering velocity of the generating and generated waves resulted insuccessive constructive and destructive action. The effect is successivecancellation producing a null point at each position at whichsuccessively generated waves are 180 out of phase. In practice, thiscondition gives rise to a coherence length" defined as the distance overwhich the two waves of concern are sufficiently close to being in phaseto result in continued amplification of the generating wave. Use ofmaterials of greater thickness in the transmission direction is notadvantageous since the generation process may be considered as startingafresh at each 180 interval.

The problem of dispersion in nonlinear devices has been approached fromdifferent directions. Easily the most popular solution involves the useof birefringent nonlinear material in which the birefringence issufficiently large that the velocity shift for different polarizationsof frequencies of concern is as large as the velocity shift due todispersion, see Vol. 8, Physical Review Letters, (1962), p. 19. Indevices in which phase matching is accomplished by use of birefringence,the polarization and wave direction of radiation incident on the mediumis such that a wave of a first frequency and polarization travels at avelocity equal to that of a second frequency and polarization. Theprinciple is applicable to phase matching of three frequencies as well.

Crystalline materials of primary concern for use in birefringencephase-matching nonlinear devices have an optic axis (along which thereis no birefringence). Maximum birefringence and, therefore, maximumdivergence of frequencies which are phase matchable result when theincident radiation is normal to the optic axis. Frequencies of lesserdivergence are phase matched for incidence angles difi'ering from 90.

Use of an incidence angle other than 90, however, results in refractiveloss of energy by walk off." The preferred conditions are, therefore,those which result in phase matching for incident radiation at 90 to theoptic axis. This condition is known as noncritical phase matching."

The first material, in which birefringence phase matching was reported,was KDP (potassium dihydrogen phosphate). This material continues to beof device interest. Commercial survival of KDP, however, is due largelyto ready availability of requisite size and quality sections as comparedto other nonlinear materials.

Two later materials, LiNbO, and a mixed crystal of barium sodiumniobate, are phase matchable over a broad frequency range and havenonlinear coefficients of far greater magnitude than that of KDP.

Of the two later materials, the most recent, barium sodium niobate,appears the more desirable by reason of its device characteristics.Growth procedures have, however, not been developed to the stagerequired for large scale production, and this material continues to bequite expensive.

LiNbO, is generally available in large sections of good crystallineperfection, and it appears from the literature that this material isconsidered by many to be a promising candidate for nonlinear use.LiNbO,, however, has a shortcoming which is considered to be significantfrom a device standpoint. It has been observed that exposure of thiscrystalline material to light intensities of the amplitude ordinarilyencountered in laser experiments soon produces local inhomogeneities inrefractive index which, since they act as light scattering centers,quickly render the material useless.

The discovery that radiation damaged LiNbO could be recovered byannealing (l2 Applied Physics Letters l86 (1968)) gave rise to thepossibility of operation at elevated temperature such that radiationdamage could be avoided. (The severity of radiation damage is intensityrelated.) To eliminate damage, operation at temperatures of the order ofat least to 250 C are indicated.

Another difficulty associated with lithium niobate, now overcome, wasconcerned with compositional variations observed to occur during growthfrom the melt. These gave rise to concomitant variations in refractiveindices and, in turn, to variations in noncritical phase-matchingtemperature. The solution to this problem came about upon observationthat a congruently melting composition resulted when the melt containeda molar ratio of Li,O/Li,O+Nb,O of 0.486. This, of course, represents adeparture from stoichiometry for the nominal composition LiNbOPhase-matching temperature (and this terminology hereafter used to meannoncritical phase-matching temperature) is dependent on birefringence asnoted and birefringence, in turn, decreases with increasing temperature.Phasematching temperature is, of course, also dependent on other factorssuch as dispersion and the frequencies of concern. The largestbirefringence in usual device use occurs for the case in which onefrequency is twice the other, i.e., for SHG or degenerate downshifting.Of course, it varies too for the particular frequencies involved.

As a relevant example, phase matching for degenerate downshifting of the5,145 A line of the argon-ion laser occurs at about 15C for crystals ofLiNbO; grown from a stoichiometric melt. The desire to operate above thedamage" temperature of, for example, about 250C is in no way advanced byuse of a congruent melt, since phase matching for the same frequenciesin crystals so produced occurs at about 65 C. Most other parametricapplications involving other frequencies require still higherphase-matching temperatures.

There is, therefore, a desire to produce a lithium niobium crystal whichmay be phase matched above the damaged temperature and which shows thehomogeneities in refractive index associated with crystals grown fromcongruent melts.

SUMMARY OF THE INVENTION Growth of lithium niobium from a congruent meltmodified by addition of magnesium oxide, MgO, results in crystals havingincreased phase-matching temperature for any given pair of frequencieswhile evidencing the uniformity in refractive index previouslycharacterized only by growth from the nominal 0.486 molar ratiocongruent melt. Generally, the amount of MgO to be added to the melt isfrom 1 to 8 mol percent based on the entire melt. Within this range,larger increases of MgO results in further increasing of phase-matchingtemperature for any given set of frequencies. This corresponds to arange of from about 1.5 mol percent to about l2 mol percent in the solidend product.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1, on coordinates of a function ofthe second power of wavelength and phase-matching temperature, is a plotshowing the relationship between these two parameters for crystals grownfrom three different melts; the first stoichiometric, the secondcongruent, and the third a congruent melt modified by the addition ofMgO in accordance with the invention;

FIGS. 2A and 28, on coordinates of second-harmonic power andtemperature, are plots illustrating the efiect of homogeneity of amaterial, in accordance with the invention, in terms of generated powerat a given wavelength. These figures are discussed in conjunction withthe description of Examples 1 and 2; and

FIG. 3 is a schematic view of a nonlinear device using a material of theinvention.

DETAILED DESCRIPTION 1. Terminology a. In accordance with conventionalpractice, the relevant material is referred to herein as lithium niobiumor simply LiNb0,. It should be understood that this is merely a nominalcomposition designation and that, by definition, neither the unmodifiedmaterial nor that modified in accordance with the invention is ever ofthat precise composition. In fact, the lithium to niobium ratio rarely,if ever, corresponds with the atom ratio indicated by the stoichiometriccompound.

b. As has been indicated, the terminology phase-matching temperature" isintended to mean noncritical phase-matching temperature or, in otherwords, the temperature at which phase matching results for a beamintroduced normal to the optic axis. Of course, phase-matchingtemperature," per se, is not meaningful without designating thefrequencies of concern. Where use is made of this terminology withoutdesignating such frequencies, it is to be understood that the use iscomparative and that it is intended only that this temperature for agiven set of frequencies follows whatever relationship is indicated.

c. "Congruently melting" composition or melt has reference to thecomposition which may be melted and refrozen without any alteration incomposition. It follows that such a congruent melt may be frozen in itsentirety without any variation in lithium to niobium ratio in the growthdirection. Insofar as is relevant to this description, such congruencyrelates only to the composition before modification by addition ofmagnesium. It is inherent in the inventive teaching (and, in fact,responsible for its value) that the modified composition is incongruent.The growing crystal shows a continuous variation in Li/Nb ratio and alsoin Mg/Li ratio. It is the essence of the invention that the distributioncoefficients are such that the compositional variations areselfcompensating in that the final product shows a consistency inrefractive index in the growth direction (of course, while havingincreased phase-matching temperature).

d. The congruent composition in the nominal system LiN- bO, has beenestablished to a precision of three decimal places. It has beenindicated that the molar ratio is 0.486 in terms of Li,OILi,O+Nb,0,.While it has been established that this is the optimum unmodifiedcomposition for the practice of this invention, a practical range of0.486 1- 0.004 is considered satisfactory for many purposes. Deviationfrom optimum does not affect the self-compensating nature of the Mg/Liratio and the variation in refractive index of the resulting product isapproximately that of the unmodified growing congruent crystal.Congruency" or similar terminology has reference to this molar ratiorange in the unmodified melt (or crystal).

2. Composition The unmodified composition has been indicated underTerminology. It is fundamentally that which has already been establishedas congruently melting (see Vol. 42, Journal of Applied Physics (Apr.1971)). It has been indicated that the of MgO to the melt. Thiscorresponds to a 1.5 percent to 12 weight percent content of M30 in thefinal product based on total composition. In actuality, the amount ofMgO (and also the relative amounts of lithium oxide and niobium oxide)constantly vary during growth. The actual composition of concern is theinitial composition (either of the melt or of the crystal) since it isthis composition which establishes the phasematched temperature.Subsequent variation in composition has no effect on phase-matchedtemperature.

Appropriate starting compositions are selected on the basis ofinformation such as that referred to by FIG. 1. It is the essence ofphase matching that different polarization waves of difierentfrequencies may travel at such velocities through the crystal as to meetthe phase-matching conditions (for the simple two-frequency case thiscalls merely for equal velocity of both polarizations). As temperatureis increased, the ferroelectric Curie point of the material isapproached. For a given incident frequency, the phase-matchingtemperature, therefore, increases as the frequency of, for example, thegenerated wave approaches that of the initial wave. It is seen,therefore, that the degenerate case (or the equivalent up conversion,i.e., SHG) requires the highest birefringence and, therefore, results inthe lowest phase-matching temperature for any given composition. (In theabove discussion, variation of dispersion with temperature has beenignored the overall effect being as though temperature dependence ofbire-fringence were controlling. In fact, dispersion does vary withtemperature but the simplified criterion is adequate for thisdiscussion.)

It follows from the above that the amount of MgO added to the melt is tobe selected in accordance with the desired phase-matching conditions.Since it is essential to the invention that operation be at atemperature in excess of at least 170C and preferably 250 C to avoidradiation damage and since increasing the amount of MgO increasesphase-matching temperature, the minimum amount required decreases as thefrequencies of the waves of concern diverge.

3. The Figures The data of FIG. 1 is plotted in the manner described inthe literature, see 17 Applied Physics Letters 104 (1970). Informationplotted on this figure is for a stoichiometric melt (curve 1), acongruent melt (curve 2), and an MgO containing melt (curve 3). Thelatter is based on a crystal grown from a composition initiallycontaining 1 mol percent MgO based on a total of the initial constants;MgO, U 0, and Nb,0,,. Based on other experiments, it has been determinedthat MgO addition increases the phase-matching temperature by an amountof approximately 27C per mol percent on the same basis. The plotted formis quite useful in that it yields phase-matching temperatures for anygiven set of wavelengths whether degenerate or nondegenerate. The methodby which such information may be determined from the curve formpresented, while described in detail in the noted reference, is brieflyset forth. The ordinate units are defined on the figure as (Ar/i.) +1 mwhere A, is the idler wavelength and A, is the signal wavelength in thesame units. The abscissa units are phase-matching temperature in degreescentigrade. The necessary phase-matching temperature is determinedsimply by selecting the desired signal wavelength and the necessaryidler wavelength in accordance with the relationship HR, I/A, l/MEquatign where A, is the pump wavelength. So, for example, for a pumpwavelength A, equal to 0.5 145 p. (micrometer) and for a signalwavelength to idler wavelength ratio of 1/3, the phasematchedtemperature is approximately 250C.

The figures denoted 2A and 2B are discussed under the secmodificationconsists of addition of from 1 to 8 percent by mol tion entitledExamples below.

In FIG. 3 there is depicted a single crystal body 11 of UN- bo Thecrystallographic orientation of the body is indicated on the figure. Acoherent electromagnetic beam 12 produced by source 13 is introducedinto body 11, as shown. The resultant emerging beam 14 is then caused topass through filter 15, and, upon departing, is detected by apparatus16. For the SHG case, beam 12 is of a fundamental frequency whiledeparting beam 14 additionally contains a wave of frequencycorresponding with the first harmonic of beam 12. Filter 15 is of suchnature as to pass only the wave of concern, in the SI-IG instance, thatof the harmonic. Apparatus 16 senses only that portion of the beamleaving filter 15. The value of 0, may be varied in body 11 by alteringthe angle between beam 12 and the Z axis, as by rotating the crystalabout the Y axis. As has been indicated, the maximum birefringence isobtained for an angle of 90 degrees.

The device of FIG. 3 may similarly be regarded as a threefrequencydevice, with beam 12 containing frequencies to be mixed or consisting ofa pump frequency. Under these conditions, exiting beam 14 containssignal and idler frequencies as well as pump, representing threedistinct values for nondegenerate operation. For any operation, whethertwo frequency or three, efficiency is increased by resonance. Such maybe accomplished by coating the surfaces of crystal 11, through which thebeam enters and exits. This coating may be partially reflecting only fora generated frequency, as for example for the harmonic in SHG. For thethree-frequency case, it is desirable to support both generatedfrequencies. In most instances, this cannot be accomplished by coatingthe face of the crystal, and it is necessary to provide at least onespaced adjustable mirror which may be positioned at such distance fromthe face of the crystal 1] as to support the frequencies of concern.Simultaneous support of the pump frequency may similarly beaccomplished. However, the complication so introduced is justified onlywhen the pump level requires it.

The crystalline o ientation shown as the initial position for crystal 11in the apparatus of FIG. 3 eliminates the effect of double refraction,as has been discussed. This angle may be retained for a broad range ofconditions when operating either in the degenerate or nondegenerate modesimply by controlling temperature.

has been indicated that the device depicted in FIG. 3 is merelyexemplary of a large class of nonlinear devices utilized as harmonicgenerators, parametric mixers, parametric amplifiers, et cetera. Such aclass of devices is described in detail in copending US. applicationSer. No. 414,366, filed Nov. 27, 1964.

4. Examples The examples yielded data which has been plotted as FIGS. 2Aand 2B. The first of the figures corresponding with Example 1 containsdata taken from a sample of LiNbO grown from a congruent melt. Thesecond of the figures (Example 2) contains data taken from an experimentconducted on a sample of the same congruent ratio but containingapproximately 1 mol percent of MgO, based on initial ingredients in themelt (this corresponded to approximately 1% mol percent of MgO on thesame basis in the initial portion of crystal drawn from the melt).

Example I A crystal of nominal composition LiNbO was grown byCzochralski growth from a melt of the composition 48.6 mol percent U 0and 51.4 mol percent Nb O A crystal of the apwith v ing temperature.

FIG. A indicates the power of the output signal as a function oftemperature peaked at 46C with a line width of 032C.

EXAMPLE 2 FIG. 28 contains similar information for an example grown froma melt such as that described above but modified by substitution of MgO.The amount of MgO included in the melt was 1 mol percent. The amounts ofother starting ingredients were as set forth in Example 1. The finalcrystal composition grown under the conditions set forth in Example 1maintained a substantially constant birefringence over a crystal sectionlength of 1.49 cm. The initial part of the crystal grown containedapproximately l.5 mol percent magnesium (with attendant reduction inlithium by about 3 mol percent). Peak power for the signal wavelength of5,400 A was at about 87C (an increase of about 4lC as compared with thecongruent crystal) and line width was about 050C. This value iscomparable to that found for the congruent crystal in FIG. 2A whenconsideration is given to the different lengths.

From these and other examples it was found that line width wassufficiently narrow for most device uses only when the initial melt wassubstantially congruent (48.6 mol percent 1 0.4 mol percent). Therelationship remained substantially unaffected for differing amounts ofmagnesium inclusion over the range of up to 12 mol percent of initialingredients in the melt on the basis indicated.

What is claimed is:

l. Phase-matched nonlinear devices for operation over the frequencyrange of from about 5 micrometers to about 0.4 micrometers provided withfirst means for introducing electromagnetic radiation containing acomponent within said wavelength range, and second means for extractingelectromagnetic radiation containing a component of a differentwavelength within said wavelength range, in which said device dependsfor its operation on a substantially single crystal body comprising thecomposition indicated by the nominal formula LiNbO said compositionhaving been grown from a melt containing U 0 and Nb O in the molar ratiooffrom 48.2:5 1.8 to 49:51, characterized in that said body is grownfrom a melt initially containing from 1 to 8 mol percent MgO based onthe said starting composition.

2. Device of claim 1 in which the MgO composition of the solid is withinthe range of from 1.5 to 12 mol percent.

3. Device of claim 1 in which said first and second means compriseoptically polished parallel flat surfaces.

4. Device of claim 2 in which said second means includes a coating sodesigned as to preferentially transmit the said second component.

5. Device of claim 1 provided with means for maintaining said body at atemperature of at least C.

6. Device of claim 5 provided with means for maintaining said body at atemperature of at least 250C.

2. Device of claim 1 in which the MgO composition of the solid is withinthe range of from 1.5 to 12 mol percent.
 3. Device of claim 1 in whichsaid first and second means comprise optically polished parallel flatsurfaces.
 4. Device of claim 2 in which said second means includes acoating so designed as to preferentially transmit the said secondcomponent.
 5. Device of claim 1 provided with means for maintaining saidbody at a temperature of at least 190*C.
 6. Device of claim 5 providedwith means for maintaining said body at a temperature of at least 250*C.